1. Field of the Invention
The present invention relates to an erbium-doped fiber amplifier pumped in the 0.98 .mu.m and 1.48 .mu.m bands and an optical fiber communication system having the same.
2. Description of the Related Art
Having many advantages over an electrical communication system, an optical fiber communication system has been actively studied and developed or already put into practical use in various fields. In such an optical fiber communication system, the propagation loss can be easily compensated by using erbium-doped fiber amplifiers which can directly amplify a light signal without converting it to an electrical signal.
Since the fiber amplifiers have a high saturated output, it is possible to increase the number of distribution ends in the entire optical system. Especially, when the fiber amplifiers are used for an optical subscriber system where information with a large capacity such as images is transmitted from a base station to home terminals or for a system called "Fiber To The Home", multi-channeling and multi-distribution can be more easily achieved. This contributes to facilitating the propagation of the system in the aspects of both the software and the cost. In the "Fiber To The Home" system, a multi-channel image signal transmission service called an optical CATV where the existing electrical CATV system has been developed to utilize optical transmission is predicted to be first put into practical use. Analog signal transmission is more advantageous for the above system than digital signal transmission because the former is compatible with the existing image information recording mode and is less expensive than the latter. Accordingly, the system adopting the analog mode is now being studied. The amplitude modulation or the frequency modulation is generally used for the analog signal transmission. In either case, the performance of the system depends on the carrier-to-noise ratio (CNR) and the distortion property. The CNR property is especially important for an AM-FDM (frequency division multiplexer) optical distribution system.
The fiber amplifier is essentially a low noise amplifier. Especially, it is known that, at 0.98 .mu.m band pumping, the noise figure (NF) degrades to the quantum limit of 3 dB because the maximum population inversion is realized at this wavelength. This noise property of the fiber amplifier when pumped in the 0.98 .mu.m band is reported, for example, in IEEE Photonics Technology Letters, vol. 2, No. 6, 1990, pp. 418-421.
It is also known that, at 1.48 .mu.m band pumping, the conversion efficiency is high because the pump wavelength and the signal light wavelength are close to each other. As a result, high output can be easily obtained. Power requirements for erbium-doped fiber amplifiers pumped at various wavelengths are reported, for example, in IEEE Photonics Technology Letters, vol. 4, No. 1, 1992, pp. 46-49.
It is considered, therefore, that a hybrid pumped fiber amplifier pumped at both 0.98 .mu.m and 1.48 .mu.m may provide low noise and high output power properties simultaneously. A study on a hybrid pumped preamplifier is reported in Electronics Letters, vol. 28, No. 17, 1992, pp. 1642-1643. A study on a hybrid erbium-doped fiber in-line amplifier is reported in ECOC'91, paper WeC9-2, 1991, pp. 585-587. The latter includes an optical isolator inserted in the middle portion of an erbium-doped optical fiber for suppressing a feedback effect. As for a hybrid erbium-doped fiber post amplifier, it is reported that the NF property degrades when 1.48 .mu.m pump light increases in The Institute of Electronics, Information and Communication Engineers, 1993 spring convention paper, No. 4, C-297. This paper proposes inserting an optical isolator in the middle portion of an erbium-doped optical fiber to prevent the lowering of the population inversion.
With the insertion of an optical isolator in the middle portion of an erbium-doped optical fiber, the output power and NF properties are improved for a fiber amplifier of a bidirectional pumping mode. This is because amplified spontaneous emission (ASE) propagating opposite to the direction of signal light is eliminated by the optical isolator. This is reported in The Institute of Electronics, Information and Communication Engineers, Optical Communication System Society, OCS92-27, 1992, pp. 9-16. However, since the ASE is suppressed under the high-input and high-output conditions required for a post amplifier, the effect of improving the properties by eliminating the ASE will be small for the post amplifier application.
An optical filter may be inserted in an end portion of a fiber amplifier pumped at 1.48 .mu.m or 0.98 .mu.m where a light signal has been amplified through an erbium-doped optical fiber for the purpose of eliminating beat noise generated by spontaneous emission. Such a fiber amplifier is reported in IEEE Photonics Technology Letters, vol. 2, No. 3, 1990, pp. 205-207.
It is obvious from the above description that a practical high-performance fiber amplifier can be obtained by utilizing the respective advantages of 0.98 .mu.m and 1.48 .mu.m pump wavelengths, thus increasing the output power and decreasing the NF of the amplifier.
The 0.98 .mu.m/1.48 .mu.m hybrid pumped fiber amplifier is theoretically considered to be effective for increasing the output power and decreasing the NF. In practice, however, when 1.48 .mu.m pump light is increased to obtain a high output power, the NF property degrades. On the contrary, when an optical isolator is inserted in the middle portion of an erbium-doped optical fiber in order to decrease the NF, the output power decreases. Thus, it is difficult to obtain a hybrid pumped post amplifier with high output power and low NF properties.
One object of the present invention is to provide a 0.98 .mu.m/1.48 .mu.m hybrid pumped fiber amplifier with high output power and low NF properties by isolating pump light using an optical component with low insertion loss such as an optical filter, by using an erbium-doped optical fiber having a small emission cross section in the 1.48 .mu.m band, or by pumping at a wavelength of the 1.48 .mu.m band capable of providing a smaller emission cross section.
Another object of the present invention is to provide a light signal transmission system with enhanced capabilities of long-distance transmission and multi-distribution.